Weber solid surface griddle advice?

It should be possible to grind it smoother with some appropriate tool. A sanding block (such as a convenient scrap of wood) and fairly fine carbide wet/dry sandpaper used wet, would be one reasonable approach. I have a couple of cast iron frypans with smooth ground interiors and one without, and I much prefer the smooth ones because they seem to stick somewhat less.

Of course, after any sort of grinding or sanding, cleaning and reseasoning is required. Cooking up a bunch of bacon helps the seasoning process along, and also good bacon is delicious.